Conventionally, lubricating oils have been used in internal combustion engines, transmissions and other mechanical devices so as to facilitate the smooth operation thereof. In particular, lubricating oils for internal combustion engines (engine oils) have been required to possess high characteristic performances due to the fact that internal combustion engines have been improved in performances, increased in power output and used under more severe operating conditions. Therefore, conventional engine oils are blended with various additives such as anti-wear agents, metallic detergents, ashless dispersants, and anti-oxidants for fulfilling these performance requirements.
Further, engine oils have been demanded to be improved in fuel efficiency from the view point of recent environmental issues concerning reduction of carbon dioxide emissions. In order to meet the demands, there have been promoted some means such as blending of friction reducing agents such as MoDTC (see Patent Document No. 1 below) or increasing the viscosity index of lubricating oils. Friction reducing agents such as MoDTC are significantly inhibited from performing the initial friction reducing effect when used in diesel engine oils which are likely to be contaminated with soot, and it is thus important to increase the viscosity index of the lubricating oil. In general, a viscosity index improver is blended with a lubricating oil so as to increase the viscosity index thereof. An olefin copolymer is less in viscosity index improving effect while a polymethacrylate viscosity index improver is high in viscosity index improving effect but poorer in thermal stability than the olefin copolymer. Therefore, it is common to blend an olefin copolymer that gives less influence on engine detergency or to reduce the amount of the viscosity index improver to be blended, in an engine oil used in diesel engines which are high in heat load and severe in engine detergency requirements due to contamination by soot. When polymethacrylate is used, it is necessary to blend large amounts of metallic detergents, ashless dispersants and anti-oxidants to maintain engine detergency. As the result, the production cost will be extremely increased and other requisite performances would be adversely affected.
That is, for diesel engine oils, it is very difficult to maintain the engine detergency at a higher level and also improve the fuel saving performance by increasing the viscosity index of the oils.
Recent diesel engines have been equipped with devices for reducing the emission of particulate matters such as diesel particulate filters (DPF). However, the diesel engine oils have been required to be less in ash content to avoid the devices from clogging. Lowering the ash content of an engine oil means decreasing the amount of the metallic detergent, and as the result, there has arisen an important issue concerning securement of the detergency for diesel engines, in particular detergency for the grooves of the top rings, which was maintained by blending large amounts of a metallic detergent and an ashless dispersant.
That is, it is assumed that a low ash diesel engine oil that can accomplish engine detergency and fuel saving performance at higher levels has not existed yet.
As the results of the extensive research and study carried out by the inventors of the present invention to improve the long-drain properties such as base number retention properties, high temperature detergency and fuel efficiency of a lubricating oil, they succeeded in improving these properties by blending phosphorus compounds such as metal salts of alkyl phosphoric acid, using no or less amount of zinc dithiophosphate (ZDTP) that has been conventionally used (see Patent Document No. 2 below), the performances specialized in base number retention properties and high temperature detergency by optimizing metallic detergents (see Patent Document Nos. 3 to 5 below) and the performances specialized in fuel efficiency by lowering the ash or phosphorus content (see Patent Document Nos. 6 to 8 below). However, there is still room for improvement in both engine detergency, in particular top ring groove detergency and fuel saving performance by increasing the viscosity index, for diesel engine oil which is likely to be contaminated with soot.    Patent Document No. 1: Japanese Patent No. 3615267    Patent Document No. 2: Japanese Patent Laid-Open Publication No. 2002-294271    Patent Document No. 3: Japanese Patent No. 3662228    Patent Document No. 4: Japanese Patent No. 3709379    Patent Document No. 5: Japanese Patent No. 3738228    Patent Document No. 6: Japanese Patent Laid-Open Publication No. 2004-035619    Patent Document No. 7: Japanese Patent Laid-Open Publication No. 2004-035620    Patent Document No. 8: Japanese Patent Laid-Open Publication No. 2004-083891